It is known to provide an evaporative emission control system for a motor vehicle in order to reduce the emissions produced by evaporation of the fuel.
In many countries regulations are in force requiring manufacturers to provide in-service detection and notification of leaks in the evaporative emission control system so as to minimise pollution.
In order to meet such regulations it is known to carry out a “natural vacuum leak detection test” (NVLD test) using a vacuum operated switch as shown in, for example U.S. Pat. Nos. 6,823,850, 7,047,950 and 7,216,636. The test uses the physical relationship between temperature and pressure which results in an increase in vacuum in a closed system if the temperature falls. Therefore, in a perfectly sealed system if the temperature falls the pressure will reduce and this reduction in pressure will result in the state of the vacuum operated switch changing thereby indicating an increase in vacuum in the system to a high vacuum state which can be used as an indicator that no leak is present. If, however, the system has a leak then air will flow into the system as the vacuum increases thereby reducing the level of vacuum that can be achieved due to the reduction in temperature and resulting in the state of the switch remaining in a low vacuum state.
In one embodiment of such a NVLD test for a system leak equivalent to a 0.5 mm diameter hole in the system, a vacuum operated switch is used in which the switch changes state at a vacuum of 2.5 mille bar (mb)[250 Pa] (−250 Pa from atmospheric) from an open state to a closed state. The vacuum at which the switch changes is known as the switching level or predetermined level of vacuum and a vacuum greater than this is known as a high vacuum and a vacuum less than the switching level is known as a low vacuum.
That is to say, if the switching level is a vacuum of 2.5 mille bar [250 Pa], then a high vacuum would be a vacuum greater than this such as four mille bar [400] and a low vacuum would be a vacuum less than the switching level such as for example 1.5 mille bar [150 Pa]. Therefore, the absolute pressure in the high vacuum range for a vacuum greater than the switching level is less than the absolute pressure in the low vacuum range which extends from the switching level to atmospheric pressure.
After engine switch off a delay of ten minutes is allowed for the system to stabilize and then the state of the switch is checked, if the switch is closed this indicates the presence of a high vacuum and the test is repeated after a time delay of ten minutes, if the switch is still closed the test is passed and no leak is considered to be present.
If upon conducting the first test of the switch, the state of the switch is open indicating low or no vacuum, the test is repeated at ten minute intervals for up to a further twenty-four hours and, provided the temperature has fallen at least six degrees Centigrade for a period of two hours, if the switch has not changed to the closed (high vacuum) state at any time during this two hour period then a leak is presumed to be present and an indication is provided to an electronic error manager for managing the illumination of a malfunction indicator lamp (MIL). It is normally the case for the error manager to operate such that two consecutive tests have to indicate the presence of a leak before the MIL is illuminated and so a single detection of a leak would not in this case result in illumination of the MIL. If at any time during the extended period of checking the switch changes to the closed (high vacuum) state for a period of ten minutes, then this is taken as an indication that no leak is present and the test is passed.
If the temperature does not fall at least six degrees centigrade during the two hour test period or the engine is not off for long enough then a no test will be the result and the MIL is not illuminated.
The inventors have found via test work that the above method, although reliable for indicating the presence of leaks of the required size, is not totally accurate in that leaks smaller than 0.5 mm can also result in a test failure. This is disadvantageous in that it results in the illumination of the MIL in circumstances where a leak of the 0.5 mm size is not present thereby requiring a user of the vehicle to unnecessarily take their vehicle in for checking and may result in expensive components being erroneously replaced thereby resulting in unnecessary cost to the manufacturer or to a user of the vehicle.
FIG. 5 shows the results of test work conducted using an evaporative emission system having no leak, a 0.25 mm diameter leak and a 0.5 mm leak (the size that must be detectable to meet US federal legislation).
In forty-nine tests of the sealed system no erroneous results were produced and the system was correctly identified as being sealed thirty-one times but there was a relatively high level (eighteen) of no results due primarily to the long length of time required to conduct the test and the fact that the minimum temperature drop of six degrees centigrade was not achieved.
In twenty-three tests of a system with a 0.5 mm leak, a leak was detected in five cases and there were no false sealed system results but there was once again a large number (eighteen) of no results.
In seventy-nine tests of a system with a 0.25 mm leak, the system was correctly identified as being sealed (that is to say having a leak less than the 0.5 mm requirement) twenty-five times, there were forty-three no results and there were eleven erroneous results where a leak was indicated but in fact the leak is less than the 0.5 mm requirement.
One explanation for these eleven erroneous can best be understood with reference to FIGS. 4A and 4B of the drawing.
In FIG. 4A there is shown the result of a test for a 0.25 mm leak when the temperature is reduced rapidly. In this case the rapid reduction in temperature produces a sudden increase in vacuum (sudden reduction in absolute pressure) and the switch output will change because the pressure will fall below the switch closure pressure.
However, as shown in FIG. 4B for the same 0.25 mm leak, if the temperature is reduced slowly the loss in pressure due to the reduction in temperature is partly compensated for by the air entering through the leak and so the vacuum in the system will not reach a high enough level to operate the switch. That is to say, the absolute pressure remains above the switching level.
The inventors have therefore established that there is a problem with the existing system in that a significant number of no results are produced and erroneous indications of a 0.5 mm leak are produced when a leak of this size is not present.
This description provides an improved method and apparatus for on-board leak testing of an evaporative emission control system.
Accordingly, the inventors herein have developed a method and system for leak testing an evaporative emission control system. In one embodiment, the method comprises: applying a vacuum from an engine to an evaporative emission system during an engine stop; and indicating a leak is present in the evaporative emission system when a vacuum in the evaporative emission system is less than a threshold amount after a threshold amount of time.
By applying engine vacuum to the evaporative emission system at engine stop and indicating a leak is present in the evaporative emission system when a vacuum in the evaporative emission system is less than a threshold amount after a threshold amount of time, the inventors have developed a faster and more reliable way to establish leaks in the evaporative emission system, at least under some conditions. For example, the present description allows a leak to be confirmed or denied within a short period of time after an engine stop without having to wait for ambient conditions to change before a determination of system leakage is made.
In another embodiment of the present description, the inventors provide for a vacuum testing method comprising: operating an engine to a generate vacuum; applying said generated vacuum to a fuel system component after a request to stop said engine; shutting down said engine; indicating a leak of said fuel system component in response to a change of a vacuum in said fuel system component while said engine is stopped.
In this way, engine vacuum may be applied to at least a fuel system component at engine shut-down so that vacuum leaks may be detected. Since the engine continues to rotate even as fuel and spark are deactivated, the engine generates vacuum in the intake manifold that may be used to leak test fuel system components after the engine is stopped. Further, the vacuum in the fuel system component and/or in the evaporative emission system may be set at a value between atmospheric pressure and engine intake manifold pressure. Thus, the system and method described herein provides for testing with a range of vacuum levels in the fuel system components and/or in the evaporative emission system.
According to another aspect of the description there is provided vacuum testing method for an evaporative emission control system of a motor vehicle having an engine wherein the method comprises using a vacuum decay test having the steps of predicting the vacuum in the system when an engine switch off event occurs, predicting for a leak of a predetermined size, a time period required for the vacuum in the evaporative emission control system to decay to a predetermined level of vacuum, checking whether the vacuum has fallen below the predetermined level and, if the vacuum has not fallen below the predetermined level, using this as an indication that a leak greater than the predetermined leak is not present in the evaporative emission control system.
The evaporative emission control system may include a vacuum operated switch having a first high vacuum state when the system is above the predetermined level of vacuum and a second low vacuum state when the system is below the predetermined level of vacuum for use in determining the presence of a leak in the evaporative emission control system and the method may comprise predicting the vacuum in the system when an engine switch off event occurs, predicting for a leak of a predetermined size, a time period required for the vacuum in the evaporative emission control system to decay sufficiently to cause a change in state of the switch from the first state to the second state, checking the status of the switch after the engine switch off event has occurred and, if the state of the switch at the end of the predicted time period is in the first high vacuum state, using this as an indication that a leak greater than the predetermined leak is not present in the evaporative emission control system.
Predicting a time period required for the vacuum to decay sufficiently to cause a change in state of the switch from the first high vacuum state to the second low vacuum state may further comprise determining the level of fuel in a fuel tank forming part of the evaporative emission control system and varying the predicted time based upon the level of fuel in the fuel tank.
Predicting a time period required for the vacuum to decay sufficiently to cause a change in state of the switch from the first high vacuum state to the second low vacuum state may comprise using a look up table to provide a predicted time period based upon predicted vacuum at engine switch off and the level of fuel in the fuel tank.
Predicting a time period required for the vacuum to decay sufficiently to cause a change in state of the switch from the first high vacuum state to the second low vacuum state may further comprise varying the predicted time based upon whether a pressure control valve forming part of the evaporative emission control system is open or closed.
The method may further comprise checking at predetermined intervals the status of the switch after the engine switch off event has occurred for the duration of the predicted time period and, if the state of the switch for the duration of the predicted time period has remained in the first high vacuum state, using this as an indication that a leak greater than the predetermined leak is not present in the evaporative emission control system.
The method may further comprise using a change in state of the switch from the first high vacuum state to the second low vacuum state as an indication that a leak greater than the predetermined leak is present in the evaporative emission control system.
If the result of the vacuum decay test indicates the presence of a leak greater than the predetermined leak the method may further comprise carrying out a natural vacuum leak detection test and using the result from the natural vacuum leak detection test as a final diagnostic output.
The method may further comprise determining whether the operating conditions of the evaporative emission control system are suitable for vacuum decay testing and, if the conditions are not suitable for testing, not conducting the vacuum decay test.
If the evaporative emission control system is determined to be not suitable for vacuum decay testing, the method may further comprise carrying out a natural vacuum leak detection test and using the result from the natural vacuum leak detection test as a final diagnostic output.
Predicting for a leak of a predetermined size, a time period required for the vacuum in the evaporative emission control system to decay to the predetermined level of vacuum may further comprise determining the level of fuel in a fuel tank forming part of the evaporative emission control system and varying the predicted time based upon the level of fuel in the fuel tank.
Predicting for a leak of a predetermined size, a time period required for the vacuum in the evaporative emission control system to decay to the predetermined level of vacuum may comprise using a look up table to provide a predicted time period based upon predicted vacuum at engine switch off and the level of fuel in the fuel tank.
Predicting for a leak of a predetermined size, a time period required for the vacuum in the evaporative emission control system to decay to the predetermined level of vacuum may further comprise varying the predicted time based upon whether a pressure control valve forming part of the evaporative emission control system is open or closed.
A first look up table may be used if the pressure control valve is closed and a second look up table may be used if the pressure control valve is open.
According to a further aspect of the description there is provided an evaporative emission control system for a motor vehicle having an engine, the evaporative emission control system comprising, a fuel tank, a carbon canister fluidly connected to the fuel tank and to an inlet manifold of the engine via a purge valve, a vacuum operated switch having a first high vacuum state when the system is above a predetermined level of vacuum and a second low vacuum state when the system is below the predetermined level of vacuum for use in determining the presence of a leak in the evaporative emission control system and an electronic controller operable to receive an output from the switch indicative of its current operating state wherein the electronic controller is operable to perform a vacuum decay test by using a prediction of the vacuum in the system when an engine switch off event occurs, a predicted time period required for the vacuum in the evaporative emission control system to decay sufficiently to cause a change in state of the switch from the first state to the second state for a leak of a predetermined size, monitoring the status of the switch after the engine switch off event has occurred and, if the state of the switch at the end of the predicted time period is determined to be in the first high vacuum state, use this as an indication that a leak greater than the predetermined leak is not present in the evaporative emission control system.
The electronic controller may be further operable to check at predetermined intervals the status of the switch after the engine switch off event has occurred for the duration of the predicted time period and, if the state of the switch for the duration of the predicted time period is determined to have remained in the first high vacuum state, using this as an indication that a leak greater than the predetermined leak is not present in the evaporative emission control system.
The electronic controller may be further operable to use a change in state of the switch from the first high vacuum state to the second low vacuum state as an indication that a leak greater than the predetermined leak is present in the evaporative emission control system.
If the result of the vacuum decay test indicates the presence of a leak greater than the predetermined leak the electronic controller may be further operable to conduct a natural vacuum leak detection test on the evaporative emission control system and use the result from the natural vacuum leak detection test as a final diagnostic output.
The electronic controller may be further operable to determine whether the operating conditions of the evaporative emission control system are suitable for vacuum decay testing and, if the conditions are not suitable for testing, the electronic controller is operable to abort vacuum decay testing and provide a no test output.
If the evaporative emission control system is determined to be not suitable for vacuum decay testing, the electronic controller may be operable to carrying out a natural vacuum leak detection test and use the result from the natural vacuum leak detection test as a final diagnostic output.
Predicting a time period required for the vacuum to decay sufficiently to cause a change in state of the switch from the first high vacuum state to the second low vacuum state may further comprise the electronic controller determining from a sensor the level of fuel in the fuel tank and varying the predicted time based upon the level of fuel in the fuel tank.
The electronic controller may be operable to use a look up table to provide a predicted time period based upon predicted vacuum at engine switch off and the level of fuel in the fuel tank.
The evaporative emission control system may further comprise a pressure control valve and predicting a time period required for the vacuum to decay sufficiently to cause a change in state of the switch from the first high vacuum state to the second low vacuum state may further comprises the electronic controller varying the predicted time based upon whether the pressure control valve is open or closed.
A first look up table may be used by the electronic controller to predict the time period if the pressure control valve is closed and a second look up table may be used by the electronic controller to predict the time period if the pressure control valve is open.